How can I add a nose to this head without messing its shape?
Your mesh is so "Low Poly" that every nev vertex will interact with the subsurf algorithm, modifying the overall shape; but you need some extra vertices for the nose; so the solution is: RETOPOlogizing.
Create a new object (I.E. a plane), set all the little buttons circled in red and start moving and extruding vertices: every time you move a vertex it will snap onto the surface underneath. So you will be able to put how many vertices you want, exactly where you want, without modifying your shape. Useful also for good deformation topology, if intended to be animated.